In prior art floatation machines, the process of separating froth concentrate from gangue involves either froth floatation, in which the source feed is pulp containing small fractions of the material to be concentrated, or froth floatation combined with froth separation, in which the source feed contains both small and large fractions of the material to be concentrated, the latter fraction being supplied into the froth layer. Froth floatation and a combination of froth floatation and froth separation are characterized by the need to monitor to a high accuracy the level and density of pulp in a chamber of a floatation machine since high control quality is difficult to attain unless the above parameters are accurately measured for a measurement error is automatically included into a static error in control operations. Furthermore, other necessary conditions are as follows: maintaining a predetermined pulp level in a chamber of a floatation machine with respect to its overflow threshold, thereby maintaining the pulp-froth layer interface and the thickness of the froth layer at a predetermined level, ensuring a desired liquid-to-gas phase ratio in the pulp supplied into a chamber of a floatation machine, the density and level of the pulp in the chamber being primarily determined by said ratio, maintaining optimal concentration in aerated pulp of frother supplied with circulating water and directly with the source feed, said parameter determining the size, degree of dispersion and the rise rate of air bubbles and, in effect, the density of aerated pulp; providing an optimal rate in supplying the source feed into the bulk of aerated pulp in a chamber of a floatation machine, changes in said rate impairing hydrodynamic characteristics of flows; discharging gangue from the chamber with minimum losses of the pulp liquid phase; and density of the pulp in the chamber.
Changes in the pulp level and, consequently, in the froth layer and also in the ratios between the solid, liquid and gas phases of the pulp are associated, firstly, with varying amounts of the source solid and water in the chamber of a floatation machine and, secondly, with varying losses of the liquid phase in discharging gangue due to changes in the quantity of the solid and in the content of large and heavy fractions therein. A change in the pulp level in the chamber of a floatation machine is also dependent on variations of the pulp density caused by a change in frother concentration in the pulp.
A change in the rate of feeding the pulp to the chamber of a floatation machine is caused by a variation of the solid-to-water ratio in the pulp. Thus, the disturbing factors affecting the froth floatation process are a change in the quantity of the solid and liquid phases of the pulp supplied to the chamber of a floatation machine, a change in the quantity of the liquid phase lost in unloading gangue due to a varying content of large and heavy fractions in gangue, a change in frother concentration in circulating water supplied to the chamber, which causes a variation of the liquid-to-gas phase ratio in the pulp and, in effect, variations of the pulp level and density in the chamber of a floatation machine.
From the aforesaid it follows that at the preset time quality control of the processes of froth floatation and froth floatation combined with froth separation presents an important connected with interrelated control of several parameters. This problem is particularly acute in the case of high-capacity floatation machines due to great sluggishness of a floatation installation comprising such machines and also because of major disturbing factors of different character.
The above problem is partially solved in a known device for controlling automatically the process of separating froth concentrate from gangue in a floatation machine (cf. G. M. Kovin et al: "Systemi avtomaticheskogo kontrolia i upravlenia teckhologicheskimi protsessami flotatsionnikh ustanovok", 1981, "Nedra" publishers, Moscow, pp. 69-73), which comprises a pulp level measuring circuit wherein a bubbling tube disposed in a chamber of a floatation machine communicates with a pressure transducer and an air flow governor. The output of the pressure transducer is connected to a pulp level recorder and to the input of a circuit designed to control the rate of unloading gangue from the chamber of the floatation machine and having its output connected to the drive of a control valve arranged on a branch pipe used to unload gangue from the chamber of the floatation machine. The foregoing device also includes a frother flow control circuit, in which the frother flow is controlled in proportion to the flow of gangue discharged from the chamber as pulp.
The disclosed automatic control device fails to provide for desired control quality due to the fact that, in measuring the pulp level in the chamber of the floatation machine, no account is taken of the error caused by changes in the pulp density, and also as a consequence of low accuracy in pulp level measurements. Its low accuracy in measurements is attributable to the presence of a constant component dependent on the depth of immersion of the bubbling tube in the pulp in the signal proportional to the pulp level. The above disadvantage is also associated with the fact that the bubbling tube is placed directly in the pulp, a factor leading to clogging of the bubbling tube in its lower portion and, in effect, distorting the measurement results and introducing an additional error.
The metering of the frother flow with respect to the flow of gangue discharged as pulp in the absence of pulp density monitoring is very approximate and impairs froth formation and hydrodynamic conditions in the chamber due to great variations of the pulp density, which generally hinders pulp level stabilization. Moreover, the absence of such a control action as water and frother flow stabilization during the floatation process makes more difficult the pulp level stabilization process.
The foregoing automatic control device does not ensure required control quality, particularly in high-capacity floatation machines due to severe disturbances and great sluggishness of floatation installations comprising such machines.
The above problem is partially solved in another known device for controlling automatically the process of separating froth concentrate from gangue in a floatation machine (cf. GB, A, 2180779), which comprises a channel used to measure the level and density of pulp in a chamber of a floatation machine, wherein two bubbling tubes installed at different levels in the pulp in the chamber of the floatation machine communicate with air flow governors and a differential pressure transducer connected to the input of a channel used to control the flow of frother supplied to the chamber of the floatation machine and to a first data input of a pulp level correction unit whose second data input is connected to a pressure transducer communicating with one of the bubbling tubes, while its output is connected to the input of a channel used to stabilize the pulp level in the chamber of the floatation machine, its circuit designed to control the rate of discharging gangue from the chamber of the floatation machine being connected to the drive of a gangue discharge valve placed on a branch pipe used to discharge gangue from the chamber of the floatation machine.
Similarly to the previously described device, the last-mentioned automatic control device fails to provide for desired control quality. This disadvantage is attributable to the fact that pulp level stabilization solely by changing the rate of gangue discharge from the chamber of the floatation machine is generally inefficient, particularly in the case of high-capacity floatation machines due to such factors as great sluggishness of floatation installations comprising such machines, the presence of strong disturbances and insufficient power in effecting control required to rapidly restore the pulp level to a predetermined value. A mere increase in the gain of the pulp level stabilizing channel with great disturbances will cause system driving, overcontrol and longer corrective action, due to which control quality will be adversely affected. The use of a more powerful actuator valve for discharging gangue from the chamber of a high-capacity floatation machine would increase its own sluggishness and, in effect, the sluggishness of a floatation installation comprising such a machine and automatic control means, and impair the control characteristics.
The aforesaid automatic control device is also incapable of ensuring the required control quality due to low accuracy in measuring the pulp density and, consequently, the pulp level in the chamber of a floatation machine since the pulp level is corrected with respect to the pulp density and the error occurring in measurements of the pulp level and density is automatically included into a static error in control operations, the former error being caused by the above factors.
In the foregoing automatic control device the rate of gangue discharge from the chamber of the floatation machine is changed by the use of an actuator valve having a nonlinear flow characteristic, which does not provide for desired control quality. Such an actuator valve fails to ensure maximum free discharge of gangue from the chamber of the floatation machine with minimum losses of the pulp liquid phase, which likewise results in disturbances as regards the pulp level in the chamber or the floatation machine. A vertical position of the above actuator valve (a horizontal position of the seat of the shut-off element) may result in partial or full clogging of the branch pipe used to discharge gangue from the chamber of the floatation machine in floatation of ore containing many large and heavy fractions and, consequently, bring about a change (decrease) in the rate of gangue discharge and additional disturbances as regards the pulp level in the chamber of the floatation machine, a feature further decreasing the control quality.
The floatation machine is known to use circulating water, in which residual frother concentration is as great as 70 to 80% of the working concentration. Changes occur in the flow of said circulating water with the source feed supplied to the chamber of the floatation machine and in losses occurring while gangue is discharged from the chamber of the floatation machine. This changes the pulp level and density, and also the hydrodynamic characteristics of the pulp flowing in the chamber of the floatation machine. Hence, maintaining the working frother concentration in the pulp in the chamber of the floatation machine merely by changing the additional feed of frother to the chamber of the floatation machine without stabilization of the flow of water and frother fails to provide, on the one hand, sufficient stabilization of water-to-air ratios in the bulk of aerated pulp and, in effect, of the pulp density and level and, on the other hand, a stable rate of supplying the source feed into the chamber of the floatation machine and, consequently, stable hydrodynamic characteristics of pulp flows in the chamber of the floatation machine.